What is the voltage divider formula?

The output voltage of a voltage divider is Vout = Vin x R2 / (R1 + R2), where Vin is the input voltage, R1 is the top resistor connected between Vin and the output node, and R2 is the bottom resistor connected between the output node and ground. When R1 equals R2 the output is exactly half the input voltage.

When is a voltage divider not accurate?

A voltage divider is only accurate when the load connected to the output draws very little current compared with the divider current. If a low-impedance load is connected, it effectively appears in parallel with R2, lowering the output voltage. As a rule of thumb the load impedance should be at least ten times the combined R1 and R2 to keep the output within a few percent of the ideal value.

How do I choose R1 and R2 values for a voltage divider?

Start with the ratio you need: R2 / (R1 + R2) = Vout / Vin. Then choose absolute values that set the divider current to a practical level. High resistance values waste less power but make the divider more sensitive to load impedance. Low resistance values are more stable under load but draw more current and waste more power as heat.

Voltage Divider Calculator

A voltage divider is one of the simplest and most widely used circuits in electronics. Two resistors are connected in series across a supply voltage, and the output is taken from the junction between them. The output voltage depends only on the ratio of the two resistors, not on their absolute values, which makes the divider useful for scaling a voltage down to a level that a microcontroller, sensor or analogue circuit can accept. This calculator takes the input voltage and the values of both resistors and returns the output voltage, the current flowing through the divider, the power dissipated in each resistor, and the total power wasted as heat. The formula is Vout = Vin x R2 / (R1 + R2), where R1 is the top resistor between the supply rail and the output node, and R2 is the bottom resistor between the output node and ground. The divider current is simply Vin divided by the total resistance R1 plus R2. Power in each resistor is current squared times resistance. A key limitation of a voltage divider is that it assumes no load current. If you connect a load to the output that draws significant current, it sits in parallel with R2 and lowers the output voltage. As a practical guide, the load impedance should be at least ten times the Thevenin resistance of the divider (R1 in parallel with R2) to keep the error below about 10 percent. Enter your values below and all results update as you type. Resistor values can be entered in ohms or kilohms as long as both use the same units.

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Input voltage Vin

R1 (top resistor)

R2 (bottom resistor)

6 V

output voltage Vout

Divider current0.6 mA

Power in R13.6 mW

Power in R23.6 mW

Total power7.2 mW

Assumes no load current at the output. Connect a load with impedance at least 10x (R1 parallel R2) for accuracy within about 10%.

How it works

The output voltage is Vout = Vin x R2 / (R1 + R2). The current through the divider is I = Vin / (R1 + R2). Power dissipated in R1 is P1 = I squared x R1 and in R2 is P2 = I squared x R2. Total power equals P1 + P2, which is the same as Vin squared / (R1 + R2). The ratio Vout / Vin is always between 0 and 1 for positive resistor values, so a voltage divider can only reduce a voltage, never increase it.

Worked example

A 12 V supply feeds a divider with R1 = 10,000 ohms and R2 = 10,000 ohms. Because R1 equals R2, the ratio is 10,000 / (10,000 + 10,000) = 0.5, so Vout = 12 x 0.5 = 6 V. The divider current is 12 / 20,000 = 0.6 mA. Power in R1 is 0.6 x 0.6 x 10,000 / 1,000,000 = 3.6 mW and power in R2 is also 3.6 mW, giving 7.2 mW total. These match the default values above.

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